Shared variable gain amplifier for WDM channel equalization

ABSTRACT

An amplification architecture for WDM receiver systems. The WDM channel grid is divided into groups of adjacent channels. A separate optical amplifier is provided for each channel with a single pump being shared among the channels of each group. The gain experienced by channels of a given group may be adjusted by varying the power of the group&#39;s pump. This approach allows equalization of received channel power such that all channels fall within the desired dynamic range. The amplification architecture may be implemented in a space-efficient manner at low cost.

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 09/876,533, filed Jun. 6, 2001, nowU.S. Pat. No. 6,697,193; which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

The present invention relates to optical communications systems and moreparticularly to amplification in optical communications systems.

The explosion of communications services, ranging from videoteleconferencing to electronic commerce, has spawned a new era ofpersonal and business interactions. As evident in the rapid growth ofInternet traffic, consumers and businesses have embraced broadbandservices, viewing them as a necessity. However, this enormous growth intraffic challenges the telecommunication industry to develop technologythat will greatly expand the bandwidth limitations of existingcommunications systems. Further improvements in optical communicationshold great promise to meet the demands for greater and greaterbandwidth.

Wavelength Divisional Multiplexing (WDM) technology permits theconcurrent transmission of multiple channels over a common opticalfiber, thus expanding available bandwidth and providing other advantagesin implementation. When it is necessary to recover data from the WDMsignal, the individual wavelength components are isolated from oneanother and converted to electrical form by optical receivers. Theseoptical receivers only operate correctly when the power level of theirinputs is within a specified dynamic range. Typically amplification mustbe provided to bring the power level of the optical signals up to thenecessary level due to losses in transmission and elsewhere.

FIG. 1 depicts a prior art approach to amplification within a WDMreceiver system 100. WDM receiver system 100 has as its input acomposite WDM signal 102 that includes, e.g., up to 200 wavelengthcomponents located on WDM channels spaced 25 GHz apart. An opticalwavelength router (OWR) 104 incorporates a first deinterleaving block106 that divides the 25 GHz grid into two grids having 100 WDM channelsat 50 GHz spacings. Deinterleaving blocks 108 and 110 then furtherdivide these two grids into four grids of 50 channels each at 100 GHzspacings. Each such grid is equipped with an amplifier 112 to bring thesignal power level up to the level needed for correct optical receiveroperation. A set of demultiplexers 114 then complete the separation ofthe WDM signal into its individual wavelength components.

Due to various wavelength-selective effects in the WDM link anddemultiplexing components, there must be a way of varying gain acrossthe overall grid. Otherwise, certain groups of WDM channels will havepower levels outside the required dynamic range. This is particularlytrue when the dynamic range is relatively narrow as is the case withhigh data rate systems where the individual wavelength components areeach modulated by 10 Gbps data streams or even higher data rate streams.Furthermore, these wavelength-selective effects are dependent on theparticular installation and will vary over time. Unfortunately, each ofamplifiers 112 may provide flat gain across the entire spectrum occupiedby the 200 channel grid with no provision for adaptive equalization.

One way to vary gain across the spectrum to assure optimal receiverperformance would be to install a variable gain optical amplifier withits own pump for each channel following demultiplexers 114. Bycontrolling the pump powers of the individual pumps, an opticalequalization function may be performed. Alternatively, a variableoptical attenuator (VOA) may be installed for each channel.Unfortunately, these approaches are both very expensive andspace-inefficient. Their expense and cumbersomeness increase further asthe number of channels increases.

What is needed is an amplification architecture for WDM receiver systemsthat provides an appropriate amount of amplification for each WDMchannel while economizing on component cost and space consumption.

SUMMARY OF THE INVENTION

One embodiment of the present invention provides an amplificationarchitecture for WDM receiver systems. The WDM channel grid is dividedinto groups of adjacent channels. A separate optical amplifier isprovided for each channel with a single pump being shared among thechannels of each group. The gain experienced by channels of a givengroup may be adjusted by varying the power of the group's pump. Thisapproach allows equalization of received channel power such that allchannels fall within the desired dynamic range. The amplificationarchitecture may be implemented in a space-efficient manner at low cost.

A first aspect of the present invention provides apparatus foramplifying a plurality of optical signals in a WDM communications systemcarrying a plurality of WDM channels. The apparatus includes a firstgroup of optical amplifiers, each of the first group of amplifiersamplifying a selected optical signal being carried by one of a firstgroup of adjacent WDM channels, a second group of optical amplifierseach of the second group of amplifiers amplifying a selected opticalsignal being carried by one of a second group of adjacent WDM channels,the first optical energy source providing pump energy to amplifiers ofthe first group of optical amplifiers, and a second optical energysource providing pump energy to amplifiers of the second group ofoptical amplifiers.

A second aspect of the present invention provides a WDM receiver systemin a WDM communications system carrying a plurality of WDM channels. TheWDM receiver system includes: a demultiplexer that receives a compositeoptical signal and isolates components thereof corresponding to theplurality of WDM channels, a first group of optical amplifiers, each ofthe first group of amplifiers amplifying a selected optical signal beingcarried by one of a first group of adjacent WDM channels, a second groupof optical amplifiers, each of the second group of amplifiers amplifyinga selected optical signal being carried by one of a second group ofadjacent WDM channels, a first optical energy source providing pumpenergy to amplifiers of the first group of optical amplifiers, a secondoptical energy source providing pump energy to amplifiers of the secondgroup of optical amplifiers, and a plurality of receivers for recoveringinformation transmitted via the plurality of WDM channels.

Further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art WDM amplification architecture.

FIG. 2 depicts a WDM amplification architecture according to oneembodiment of the invention.

FIG. 3 depicts the division of the WDM channel grid into subgroups foramplification purposes according to one embodiment of the presentinvention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

According to the present invention, in a WDM receiver system,amplification pump resources may be shared among a group of WDMchannels. Each such group of channels typically corresponds to a subbandof an overall WDM channel grid. Gain may be controlled on agroup-by-group basis. This approach has been found to provide sufficientequalization to assure that signal powers on individual WDM channelsremain within receiver dynamic range, even for the narrower dynamicranges demanded by very high data rate systems, e.g., >10 Gbps.

The present invention will now be described with reference to aparticular example WDM system. In the example WDM system, a 200 channel25 GHz grid is used. Receiver dynamic range extends from −10 dBm to 0dBm and the power per channel at the demultiplexer output is on theorder of −18 dBm per channel. An amplification of 8–12 dB is desired.The present invention is, however, not limited to any particular numberof channels, spacing between channels, or desired amplifier gain.

FIG. 2 depicts a WDM amplification architecture according to oneembodiment of the present invention. A WDM receiver system 200incorporates amplification features according to the present invention.The input signal 102, optical wavelength router 104, deinterleavingblocks 108, 106, and 110, and demultiplexers 114 are similar to thelike-labeled elements in FIG. 1. However, in FIG. 2, there is nocounterpart to amplifiers 112 which in FIG. 1 separately amplified each50 channel grid. Instead, each of the outputs of demultiplexers 114 hasan associated optical amplifier 202. For ease of depiction, only twoamplifiers 202 are shown for each of demultiplexers 114.

In one embodiment, each of amplifiers 202 is, e.g., an Erbium dopedfiber amplifier (EDFA). Thus there is an active fiber coupled to each ofthe outputs of demultiplexers 114. These active fibers may be doped withelements other than erbium. For example, the active fibers may be dopedwith both erbium and ytterbium. The configuration of the active fiber isoptimized in accordance with the band position of the channel to beamplified.

For each of the WDM channels, there is an associated optical receiver204 coupled either directly or indirectly to the associated amplifier202. For ease of illustration, only one optical receiver 204 isdepicted. Optical receiver 204 incorporates a photodetector thatconverts an optical signal at a given WDM channel into an electricalsignal as well as electronic circuitry to recover data from theelectrical signal.

In accordance with the present invention, the WDM channel grid isdivided into 25 groups of 8 channels each. Each such group spans asubband of the WDM channel grid. A common pump is used for each suchgroup of 8 channels. For each group of eight amplifiers there is asingle pump 206. The pump energy output by each of pumps 206 is splitamong the eight amplifiers 202 by a pump separator, implemented forexample, by a splitter 208. For ease of depiction, only 2 pumps 206 and2 splitters 208 are shown in FIG. 2. A typical pump power value for pump206 may be, e.g., 250 mW assuming use of a 1×8 pump separator. In thisway a gain of 8–12 dB may be achieved.

FIG. 3 depicts a portion of the 200 channel grid containing 32 channelsrepresenting four groups. Each channel is marked either A, B, C, or D toindicate which of demultiplexers 114 outputs the labeled channel. Theseletter designators are also shown on the demultiplexers 114 in FIG. 2.Each channel is further marked with a label px where x denotes which ofpumps 206 provides pump energy to amplify that channel. In this example,the pump requirements of each amplifier 202 are met by splitters thatdivide pump power into 8 paths for distribution to 8 amplifiers. Sincethis example also uses four demultiplexers 114, each pump 206 providespump energy to two of the active fibers output by each demultiplexer. Itwill be seen that because there are twice as many pumps 206 as there aredemultiplexers 114 in the depicted example, each pump provides pumpenergy to two of the active fibers output by a given demultiplexer.

A gain control block 210 sets the pump output power for each of thepumps 206. In this way, gain may be set for each of the subbands of theoverall WDM channel grid by simply varying the drive current of therelevant pump. The gains set for each subband may be based on a powerlevel measurement made within the receivers 204 of the correspondinggroup. The gain for each group will be set so that the input signalpowers to the receivers 204 are maintained within the desired dynamicrange. The gain control system takes advantage of the well-knownproperty that the gains within a sub-band in a WDM system measured aftera chain of optical components, including optical amplifiers, are likelyto be closely correlated. It will thus be appreciated that the schemedescribed in FIG. 2 represents a savings not only in the number ofcomponents such as optical attenuators and/or pumps that are used butalso in the additional gain control circuitry that would otherwise beneeded for each WDM channel.

The scheme that has just been described scales very well with increasingWDM channel counts and grid density. Yet another advantage of thepresent scheme is that the capacity of an existing system may beexpanded in a modular fashion. If an additional 8 channels are desired,one need only add 8 receivers, 8 active fibers and a single pump. Also,sufficient power margin may be provided to allow for per channelchromatic dispersion compensation.

It is understood that the examples and embodiments that are describedherein are for illustrative purposes only and that various modificationsand changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims and their full scope ofequivalents.

1. In a WDM communication system carrying a plurality of WDM channels,apparatus for amplifying a plurality of optical signals, said apparatuscomprising: a first group of variable gain optical amplifiers, each ofsaid first group of amplifiers amplifying a corresponding one of a firstgroup of adjacent WDM channels, said first group of amplifiers receivingpump energy from a first optical energy source; a second group ofvariable gain optical amplifiers, each of said second group ofamplifiers amplifying a corresponding one of a second group of adjacentWDM channels, said second group of amplifiers receiving pump energy froma second optical energy source; and wherein an output power of at leastone of said first optical energy source or said second optical energysource is adjusted to provide equalization to at least one of said firstgroup or second group of WDM channels.
 2. The apparatus of claim 1further comprising: a demultiplexing system that separates said channelsinto a plurality of interleaved grids, wherein said first group of WDMchannels and said second group of WDM channels each comprise WDMchannels from more than one of said interleaved grids.
 3. The apparatusof claim 2 further comprising: a first splitter that divides said pumpenergy from said first optical energy source among said first group ofoptical amplifiers; and a second splitter that divides said pump energyfrom said first optical energy source among said second group of opticalamplifiers.
 4. The apparatus of claim 3 wherein said first group ofoptical amplifiers comprises a group of active fibers.
 5. The apparatusof claim 4 wherein said active fibers are doped with one or morerare-earth elements.
 6. The apparatus of claim 5 wherein said one ormore rare-earth elements comprise erbium.
 7. The apparatus of claim 5wherein said one or more rare earth elements comprise erbium andytterbium.
 8. In a WDM communication system carrying a plurality of WDMchannels, a WDM receiver system comprising: a demultiplexer thatreceives a composite WDM signal and isolates components thereofcorresponding to said plurality of WDM channels; a first group ofvariable gain optical amplifiers, each of said first group of amplifiersamplifying a corresponding one of a first group of adjacent WDMchannels, said first group of optical amplifiers receiving pump energyfrom a first optical energy source; a second group of variable gainoptical amplifiers, each of said second group of amplifiers amplifying acorresponding one of a second group of adjacent WDM channels, saidsecond group of optical amplifiers receiving pump energy from a secondoptical energy source; and a plurality of receivers for recoveringinformation transmitted via said plurality of WDM channels; and whereinan output power of one of said first optical energy source or saidsecond optical energy source is adjusted to provide equalization to saidWDM receiver system.
 9. The apparatus of claim 8 further comprising: afirst splitter that divides said pump energy from said first opticalenergy source among said first group of optical amplifiers; and a secondsplitter that divides said pump energy from said first optical energysource among said second group of optical amplifiers.
 10. The apparatusof claim 7 wherein said first group of optical amplifiers comprises agroup of active fibers.
 11. The apparatus of claim 10 wherein saidactive fibers are doped with one or more rare-earth elements.
 12. Theapparatus of claim 11 wherein said one or more rare-earth elementscomprise erbium.
 13. The apparatus of claim 11 wherein said one or morerare earth elements comprise erbium and ytterbium.
 14. In a WDMcommunication system, a method for amplifying a plurality of opticalsignals, said method comprising: demultiplexing a WDM signal to isolatesaid plurality of optical signals from one another and outputting saidplurality of optical signals into a plurality of active fiberscorresponding to a plurality of WDM channels; receiving pump energy froma first pump into a first group of said active fibers corresponding to afirst contiguous set of said WDM channels to cause amplification withinsaid first group of active fibers; receiving pump energy from a secondpump into a second group of said active fibers corresponding to a secondcontiguous set of said WDM channels to cause amplification within saidsecond group of active fibers; and adjusting pump power of at least oneof said first pump and said second pump for equalization among saidplurality of WDM channels.
 15. The method of claim 14 whereindemultiplexing comprises separating said channels into a plurality ofinterleaved grids, wherein said first contiguous set of WDM channels andsaid second contiguous set of WDM channels each comprise WDM channelsfrom more than one of said interleaved grids.
 16. The method of claim 15wherein said active fibers are doped with erbium.
 17. In a WDMcommunication system, apparatus for amplifying a plurality of opticalsignals, said apparatus comprising: means for demultiplexing a WDMsignal to isolate said plurality of optical signals from one another andoutput said plurality of optical signals onto a plurality of activefibers corresponding to a plurality of WDM channels; means forreceiving, from a first pump, optical energy into a first group of saidactive fibers corresponding to a first contiguous set of said WDMchannels to cause amplification within said first group of activefibers; and means for receiving, from a second pump, optical energy intoa second group of said active fibers corresponding to a secondcontiguous set of said WDM channels to cause amplification within saidsecond group of active fibers; and wherein said means for receiving fromsaid first pump comprises means for adjusting pump power forequalization among said plurality of WDM channels.
 18. The apparatus ofclaim 17 wherein said means for demultiplexing comprises means forseparating said channels into a plurality of interleaved grids, whereinsaid first contiguous set of WDM channels and said second contiguous setof WDM channels each comprise WDM channels from more than one of saidinterleaved grids.
 19. The apparatus of claim 18 wherein said activefibers are doped with erbium.